Powering long LED strip, injecting power. Correct way to fuse?

Question

Recently I asked an opinion based question regarding LED strips. I have learned my lesson and now I have an actual question regarding "how power flows".

I'll explain: I have a lighting project that has 6,6 meters of LED strip and power is injected at the beginning of the strip and at the end of the strip, it goes in an almost complete loop. That long piece of LED strip has a maximum draw of 10,xx A @ 12 volts, but for simplicity let's say 10A. I never intend to drive the LED's at full brightness, so unless I code a bug in my controllers software, it should never exceed 10A anyway. Now for the actual question:

If I fuse the wiring so that there is one 10A fuse right after the PSU and 5A fuses right before injecting the power to the strip at both injection spots, is that conceptually correct? My idea is that half of the strip would be powered on each end of the strip and I assume it would be safer.

Does the situation change if I cut the strip at the middle point and only connect data and ground at that point, and power both halves individually?

• Yellow=data, red=12v, black=ground
• If necessary, I'll provide a better picture

Edit1: Or if someone can suggest a better solution altogether. I am being very neurotic about the safety of this installation.

Edit2: I thought about it a little while today and I have a hunch the below version would be better when it comes to being sure only 5A goes through each splice, maybe I could even get away with using smaller wire safely this way?

As I mentioned, if anyone has a better way to achieve this result, I'm open to ideas.

Answer 1

Wiring fuses are used to prevent wires from overheating and melting insulation or starting a fire. In that case, the key thing is that the wire diameter be large enough to conduct the maximum current the fuse will pass. Copper wire will not melt until it gets to a very, very hot and unsafe temperature. So the focus here is on the electrical insulation around the wire. The maximum current is obtained from an "ampacity chart." The ampacity depends on the temperature rating of insulation on the wire.

For installations subject to the National Electric Code (NEC) you can use an NEC ampacity table. Outside the USA I am sure there is a similar table. But in your case, they don't cover small wires and small loads, so it won't be much use.

I found a table online here: https://www.southwire.com/medias/sys_master/pdfs/pdfs/hb7/hc9/8872277901342/SW-1003267-Lead-Wire-Maximum-Current-Capacity-Whitepaper-LO-1-.pdf

It is in a whitepaper published by Southwire Corporation. The title is "Lead Wire Maximum Current Capacity."

Looking at the table, I would say you are safe using 18 AWG wire with 5 or 10 Amp fuses as long as the wire is not routed through some type of thermal insulation or something that prevents it from cooling down.

You should read the whole paper. It has more than just the ampacity chart.

Another important concept is providing some margin. If your expected load is 5 Amps, you would usually not use a 5 A fuse. You would use a slightly larger fuse, perhaps 7.5 Amps, to prevent nuisance blowing of the fuse. In that case you would want to make sure the wire can carry 7.5 Amps without over-heating. The fuse and wire always must be considered together.

If you are worried about faults in the strip itself, it would be best to be explicit about what fault you are contemplating and what kind of failures are acceptable. For sure a simple short-circuit will blow a 5 or 7.5 A fuse (assuming the power supply does not have short-circuit current limiting or protection).

Device designers often put fuses inside products to mitigate specific failure scenarios. But the details can vary quite a bit, and would likely be tested by simulating or inducing the specific failure first without a fuse, and then adding the fuse if needed. For example, if you short out a power rail capacitor, what happens? If a trace overheats and starts to smoke, you might choose to put a fuse somewhere in series with the power input for the regulator to prevent that from happening. But that is all very device-specific.

So in summary, wiring fuses are based on current carried by the wire and the wire diameter. Device fuses are based on specific knowledge of the device and specific failure scenarios, and will usually involve testing of some sort. For best user experience, fuses should be de-rated to make sure they do not blow during normal operation. However, the wiring diameter must be based on the nominal fuse rating, not the normal operating current.

Answer 2

If the cable lengths are symmetric the setup will work. Cable resistance will rise with temperature, which will help in balancing the current.

Generally the fuses should be sized to prevent fire in failure modes, which is difficult to achieve in this case as a (semi-)shorted LED would present a hazard even if the current stays within rating. Undersized fuses (5 A vs 10 A) on both ends will protect from the situation where one power line is cut.

Cables should in this case be sized for 10 A, although it's quite certain that the LED strip itself is going to be where the overheating occurs.

Check the fuse datasheets for tripping time on rated current if you expect to get close to the rating.